In recent years, considering environmental issues, attention has been paid to a vehicle employing an electric motor as a source of drive force, such as an electric vehicle, a hybrid vehicle, and a fuel cell vehicle. Such a vehicle is equipped with a power storage unit implemented for example by a rechargeable battery for supplying electric power to the electric motor and converting kinetic energy to electric energy during regenerative braking.
In such a vehicle employing an electric motor as a source of drive force, it is desirable to further increase a battery capacity of a power storage unit to improve acceleration performance and running performance such as a maximum traveling distance. As a method for increasing a battery capacity of a power storage unit, a configuration equipped with a plurality of power storage units has been proposed.
For example, U.S. Pat. No. 6,608,396 discloses an electric motor power management system providing a high-voltage vehicle traction system with a desired high direct current (DC) voltage level. The electric motor power management system includes a plurality of power stages connected in parallel and each having a battery and a boost/buck DC-DC converter for supplying DC power to at least one inverter, and a controller controlling the plurality of power stages such that the plurality of power stages can maintain a voltage output to the at least one inverter by uniformly charging/discharging the batteries of the plurality of power stages.
In the electric motor power management system disclosed in U.S. Pat. No. 6,608,396, it is disclosed that each battery is actively maintained to be in the same SOC (State of Charge) as that of other batteries in the system. To implement such a configuration, the batteries are required to have the same battery capacity.
Meanwhile, to increase a battery capacity, it is important to make good use of a space such as in a vehicle interior, and providing a plurality of power storage units having battery capacities different from each other depending on a space is also under consideration. When a plurality of power storage units having battery capacities different from each other are provided as described above, it is desirable to provide a voltage conversion unit having voltage conversion capability (such as allowable conversion power, an allowable conversion current value, and a voltage conversion possible range) suitable for the battery capacity (or charge/discharge current) of each power storage unit, to reduce loss due to voltage conversion.
Such voltage conversion units having voltage conversion capabilities different from each other inevitably have different response characteristics, due to differences in inductance and the like. Therefore, if the electric motor power management system disclosed in U.S. Pat. No. 6,608,396 is directly applied, a voltage difference may be caused between the voltage conversion units during transition such as immediately after system start-up, and an unwanted circulation current may flow between the power storage units through the voltage conversion units. Such a circulation current may damage the power storage units.